Evidence based on four field surveys conducted between July 2009 and April 2011 indicates that both sea surface partial pressures of CO2 (pCO(2)) and sea-air CO2 fluxes at Luhuitou fringing reef in Sanya, Hainan Island, northern South China Sea (SCS) are subject to significant seasonal variations. The diurnal variation of seawater pCO(2) ranges from 264 to 579 mu atm in summer, which is much larger than that in autumn (152-335 mu atm), in winter (84-260 mu atm), and in spring (114-228 mu atm). The sea-air CO2 flux in summer (similar to 9.6 mmol CO2 m(-2) d(-1)) is also larger than that in other seasons (i.e.,similar to 3 mmol CO2 m(-2) d(-1) in spring, similar to 3.5 mmol CO2 m(-2) d(-1) in autumn, and similar to 2.7 mmol CO2 m(-2) d(-1) in winter). The atmospheric pCO(2) in this reef shows small diurnal and seasonal variations. The integration of the time-series pCO(2) data shows that the reef area is a weak source of atmospheric CO2 at similar to 0.54 mol CO2 m(-2) yr(-1). Further analyses indicate that the seasonal variations of the surface seawater pCO(2) in Luhuitou fringing reef are mainly affected by seasonally-dependent biological metabolic processes (organic processe and inorganic process), and that the organic process play a more important role than the inorganic process. Seasonal sea surface temperature (SST) variations and hydrodynamic processes may also have some influence on seawater pCO(2) variation.